The present invention relates to a system and method of controlling a vehicle steer-by-wire system to provide variable and adjustable steering feel for a driver, wherein the system utilizes expert-type fuzzy logic technology to improve vehicle handling performance and safety.
One vehicle attribute that can improve vehicle handling performance and safety is a steering feel for the driver. Steering feel represents a general relationship between the torque at the steering wheel and the force at the road wheel tire-road surface contact. The force at the road wheel tire-road surface contact should be fed back to the steering wheel to produce steering wheel torque felt by the vehicle driver. This steering torque feedback allows the vehicle driver to rely on the steering feel to sense and maintain control of the direction of the vehicle. If the steering torque feedback is not present and appropriately tuned for the vehicle application, then the driver may experience vagueness in feel and degradation in overall directional controllability.
A common current production steering system is a power-assisted steering system with mechanical linkages. Such systems provide a determinate steering wheel torque curve based on their mechanical and hydraulic arrangement. In a steer-by-wire system that does not rely on a mechanical connection between the steering wheel and the road wheels, the requirement is to produce not only the same steering functions and steering feel as a conventional mechanically linked steering system, but also advanced steering features including a variable and an adjustable steering feel. Thus, different steering feel can appear in the same vehicle based on driver choice.
Steer-by-wire systems have been challenged in providing the advanced steering features including a variable steering feel and an adjustable variable steering feel. Such advanced steering features with variable and adjustable steering feel can be implemented by applying advanced control strategies and control system design. The electric motor-based steer-by-wire control system provides a flexible application environment for control system designers to use more sophisticated and advanced control strategies in order to achieve the expected steering features. Some of the steering features include providing steering feel, steering wheel command angle, active steering wheel return, and road wheel tracking for steering wheel command.
The improvement of handling performance and operational safety could benefit from the professional vehicle driver""s necessary knowledge and experiences. Based on knowledge and experiences, a professional driver relies on the steering feel to sense and maintain control of the direction of the vehicle while making judgments regarding constant changes of surrounding circumstances and making predictions regarding driving circumstances. It is a challenge to incorporate the feelings, judgments, predictions and impression of a professional driver into steer-by-wire control system to realize objective of variable steering feel. It is difficult to realize this objective by using conventional control system design technologies that deal with crisp input and output physical variables and need a distinct relationship among all physical variables quantitatively, as well as require exact controlled system mathematical models. Furthermore, the conventional control techniques are unable to account directly for the difference or diversity in subjective steering feel requirements among individual vehicle drivers.
The present invention generally provides a system and method of controlling a vehicle steer-by-wire system to provide the variable steering feel for the driver utilizing expert-type fuzzy logic technology. Applying the fuzzy logic description, feelings, judgments, predictions and impression of a professional driver are incorporated directly into a steer-by-wire control system to realize objectives of variable and adjustable steering feels.
The steer-by-wire system includes two different parts according to their functions: a steering wheel control system and a road wheel control system. The steering wheel control system provides steering feel for the driver, steering wheel angle command to road wheel control system, and active steering wheel return. The road wheel control system provides the actual road wheel angle tracking to the steering wheel command angle. The steering wheel control system and the road wheel control system are in electrical communication each other. The steering wheel control system further includes a fuzzy logic controller to receive a steering wheel angle signal from the steering wheel system, a road wheel angle signal and a road wheel torque signal from the road wheel control system and vehicle speed signal from the vehicle. The fuzzy logic controller controls steering wheel torque to produce the variable steering feel continuously based on changes in steering wheel angle, road wheel torque and vehicle speed by applying fuzzy logic control technology.
In one embodiment, the fuzzy logic controller incorporates the experience and knowledge of professional drivers into an expert-type steering wheel control system to infer a desired steering feel. These experience and knowledge can be expressed in fuzzy logic inference rules in fuzzy logic controller using the natural language description. These fuzzy logic inference rules provide difference and diversity for steering feelings of individual drivers. Thus, the fuzzy logic controller design does not require mathematical models of the controlled steering system to realize predictive judgments using the experience and steering system tuning knowledge of professional drivers.
This invention describes the fuzzy logic based steering wheel torque control method implemented in the fuzzy logic controller. The fuzzy logic based steering wheel torque control operates in three steps: fuzzification, inference, and defuzzification. All crisp input and output variables including steering wheel angle error signal, road wheel torque signal and vehicle speed signal and are converted into values in the fuzzy sets by defining labels and membership functions. Then, using labels and membership functions as defined in the stage of fuzzification, a set of rules for the fuzzy inference stage are given to describe required steering wheel torque output based on the experience and steering system tuning knowledge of professional drivers. The steering wheel torque in a linguistic value description is then converted to a crisp value in the stage of defuzzification.
The present invention also describes a new development of variable steering torque control with an adjustable variable steering torque control function. The steering feel will not only vary with the steering wheel angle, road wheel torque, and vehicle speed, but it will also vary according to the type of driving and environmental situation. The adjustable variable torque function could be set by the vehicle driver or could be set automatically based on the type of driving and environmental situations. The vehicle variables that cause the steering wheel torque adjustment can be detected using sensors and estimated using other variables. This provides adjustable steering feel automatically and adaptively according to the vehicle driver""s predictive judgment for driving condition changes.
Further aspects, features, and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.